There is an increased demand for fluid filters used in personal respirators and breathing apparatuses, air filtration systems and other fluid handling systems.
A common method of filtering fluid-borne particulates or other fluid-borne materials from a fluid medium is the placement of a filter member, such as a porous screen or mesh element, directly in the fluid medium. As the fluid medium passes through the filter member, the filtered particulates are typically captured on the filter member. As the quantity of filtered material accumulates on the filter member, a load builds on the filter member, decreasing the filter member's efficiency and increasing the resistance to fluid flow through the filter member. It is a routine procedure to check for the load on the filter member by a measuring means that shows an increase of the filter resistance proportional to the load. Many prior art devices for determining the load condition of filter members are based on sensing a differential in the fluid medium pressure (for example between the “upstream” and “downstream” sides of the filter member) and creating a visual or audible signal indicative of the level of the sensed load. However, these prior approaches typically require significant pressure differences in order to operate properly, and such significant pressure differences are often extremely difficult, or even impossible, to achieve with many filtration systems that deal with only millimeters of water column, differential pressure.
Some prior art devices for indicating the filter medium load condition are directed towards systems having low aerodynamic resistances. An example of one of these devices is disclosed in U.S. Pat. No. 4,336,038 which teaches a filtering device incorporating a filter medium load condition indicator that targets, and indeed can only operate as disclosed with, specific contaminants. In this prior device, oil-containing aerosols eventually dissolve an oil soluble dye to change the color of the filter as the filter life expires. U.S. Pat. No. 4,955,995 discloses a filtering system and filter medium load indicator that employs a similar technique.
U.S. Pat. No. 3,635,001 discloses an air conditioner in which the load condition of a filter member is indicated by color contrast between the buildup of a filtered material and a free surface. This prior device allows a user visually to identify the charge on the filter, but depends strongly on the color difference between the filter material and the filtered particles, which, in the case of domestic dust, are very small.
U.S. Pat. No. 6,110,260 discloses a prior device that uses a similar approach, namely it relies upon the color difference between a buildup of filtered particulates and a filter surface. In this prior device, a change indicator includes an air impervious patch secured to the filter. A portion of the filter media adjacent to the change indicator changes appearance in response to an accumulation of contaminants at a much slower rate than the remainder of the air filter. The accumulation of contaminants progresses inwardly from the region adjacent the periphery of the change indicator towards the region adjacent to the center. The appearance of the filter media beneath the change indicator fully changes from the periphery to the center indicative that the air filter needs to be cleaned or replaced. A drawback to these prior systems is that they are only effective if the dust color is significantly different from the color of the filter. Another drawback is the necessity to observe the filter under the shielding grid that protects the device; thus, the grid has to be open for effective observation.
U.S. Pat. No. 6,375,725 discloses a device that uses contemporary optoelectronic elements to indicate the status of a filter. A problem of such prior devices is that the light generator and detector, along with the other necessary circuit board elements, increase the cost of such devices. Thus, this prior system is expensive to use and cannot be readily deployed in many everyday filtration systems.
In most cases, such as personal breathing devices, air supplying systems (air conditioners and similar), and other technological and/or technical applications using disposable filters, the life of a filter is prematurely reduced by very conservative recommended time of change or personal judgment. The prior art does not include an inexpensive device or means for observing and accurately determining the true end of service life of fluid filters.